Audio matrix encoding and decoding schemes are generally well known. Early “surround sound” technologies employed a 4:2:4 process in which four discrete audio channels are encoded (or downmixed) into two channels, passed through a two channel medium (e.g. an LP record), and subsequently decoded (or upmixed) back to four channels before being presented to four speakers. The 4:2:4 process often entails information loss, i.e., the four channels reproduced at the final stage are not necessarily identical to the four initially encoded channels. When a matrix decoder is not available, the two encoded audio channels can be played using a standard two channel stereo player.
Examples of surround sound systems employing the 4:2:4 process include the SQ (Stereo Quadraphonic) system introduced by CBS in 1971 and the QS (Quadraphonic Stereo) system by Sansui also introduced in 1971. Both the SQ and QS systems use speakers in a square configuration: left front; right front; left back; and right back. Another example of a system employing the 4:2:4 process is the Dolby Surround system introduced in 1982. This system uses speakers in a diamond configuration: left front; center; right front; and one or more rear speakers referred to as “surround” speakers.
The Dolby Surround system uses an encoding matrix known as the Dolby MP (Motion Picture) Matrix which encodes four channels of audio into a standard two channel format suitable for recording and stereo transmission. The Dolby Surround decoder recovers the four audio channels from the two encoded channels using a (4×2) decoding matrix. Since the coefficients of the decoding matrix are constants, the Dolby Surround decoder is generically referred to as a “passive” decoder. Such a decoder may provide in the range of a 3 dB separation between adjacent channels, e.g., between left and surround channels.
Passive decoders are limited in their ability to spatially orient sounds with precision for various listening positions. Active decoders, on the other hand, adapt the decoding matrix coefficients to enhance the directionality of sounds. These decoders reduce the crosstalk between channels and increase channel separation. An example of active decoders is the Dolby Pro Logic decoder introduced in 1987.
The Dolby Pro Logic II matrix decoding technology was introduced in 2000. This decoder extracts five channels of audio from a stereo (i.e. two-channel) signal: left front; center; right front; right back; and left back. The Dolby Pro Logic II decoder is suitable for use with current 5.1 surround systems, where the “0.1” refers to the reduced bandwidth low frequency effects (LFE) channel. The Dolby Pro Logic II encoder downmixes five channels of audio (or six with LFE) into two channels which can be played over a standard stereo player if no decoder is available.
Presently known active matrix decoders employ “ad hoc” schemes for computing their associated gain values, rendering them unnecessarily complex and inaccurate.